1. Field of the Invention
The present invention relates to an ink composition that can realize good images, especially waterfast images, and more particularly to an ink composition that is preferably used for ink jet recording.
2. Background Art
Ink jet recording is a printing method wherein droplets of an ink are ejected and deposited onto recording media such as paper to conduct printing. According to this method, images having high resolution can be printed at a high speed by means of relatively inexpensive apparatuses. Further, also for color images, sharp, high-quality recorded images can be easily obtained. In recent years, improved waterfastness has been required of images produced on recording paper by ink jet recording, and various combinations of water-soluble cationic resins with anionic dyes have been studied as water-base inks for ink jet recording that can realize waterfast images.
In order to impart waterfastness through addition of a cationic resin to an ink, Japanese Patent Laid-Open No. 119280/1987 discloses an ink comprising a hydroxyethylated polyethyleneimine polymer and a dye component. This publication describes that polyethyleneimine, when used in combination with a direct dye, an acid dye or a reactive dye, can develop waterfastness of the print.
Japanese Patent Laid-Open Nos. 255876/1990, 296878/1990, and 188174/1991 disclose an ink composition comprising a polyamine, with a molecular weight of not less than 300, having a primary amino group, an anionic dye, and a stability-imparting agent, such as urea or thiourea. These publications describe that the combination of the primary amine with the anionic dye can develop waterfastness in the printed ink.
Japanese Patent Laid-Open No. 305011/1995 discloses a water-base ink comprising a basic water-soluble polymer, an anionic dye with the counter ion being a volatile base, and a buffering agent with the counter ion being a volatile base. This publication describes that, in the ink, the volatile base prevents the dissociation of the polymer and, after printing, the volatile base on paper is evaporated to allow a salt-forming reaction between the polymer and the dye to proceed, thereby developing waterfastness.
According to the method wherein a cationic resin is added to an ink containing an anionic dye to impart waterfastness, the development of the waterfastness in printed images is considered attributable to a reaction of an anion of the dye with a cation of the resin on recording media. In the state of ink with a satisfactory amount of water being present therein, both the dye and the resin are in the state of dissolution in the ink. In this system, the equilibrium is significantly leaned toward the state that any precipitate does not occur. On the other hand, after printing, when the ink is dried to some extent to reduce the water content, the equilibrium is significantly leaned toward the state that the dye is reacted with the resin to create a precipitate. This renders the image waterfast.
As compared with inks free from any cationic resin, the above inks poses problems that, in some cases, satisfactory recorded images cannot be produced on specialty recording media such as glossy papers and glossy films, of which recorded images having quality equal to or higher than color photographs are expected. In particular, there are some problems including disappearance or reduction of gloss, lowered color development, lowered ink penetration. To the best knowledge of the present inventors know, the deterioration in images is attributable to the fact that the ink compositions of the above system are significantly influenced by pH and the like on the surface of recording papers.